<?php
class SF_Catalog_Model_Config extends Mage_Catalog_Model_Config
{
    public function getAttributeUsedForSortByArray()
    {
        return array_merge(
            parent::getAttributeUsedForSortByArray(),
            array('qty_ordered' => Mage::helper('catalog')->__('Orders'),
                    'updated_at' => Mage::helper('catalog')->__('Newest'),
                    'discount' => Mage::helper('catalog')->__('discount'),
					'rating_summary' => Mage::helper('catalog')->__('Rating'),
            )
        );
    }
}